1. Technical Field
The present invention relates to a method for fabricating a liquid crystal display device, and more particularly, to a method for fabricating a color filter substrate, using a small number of masks.
2. Description of the Related Art
A liquid crystal display (LCD) device displays an image using a liquid crystal driven according to an applied signal. The LCD includes an upper substrate and a lower substrate.
In general, the upper substrate is a color filter substrate for displaying an image in color, and the lower substrate is a TFT array substrate on which unit pixels are arranged in a matrix configuration.
A structure of an LCD panel comprising the upper substrate 150 and the lower substrate 100 will now be described with reference to FIG. 1.
As shown in FIG. 1, a plurality of gate lines 101 are arranged parallel to one another on the lower substrate 100. In addition, a plurality of data lines 102 are arranged parallel to one another and perpendicularly to the gate lines 101 on the lower substrate 100. Unit pixel regions are defined by intersection of the gate lines 101 and the data lines 102, and the defined unit pixels are arranged on a TFT array substrate in a matrix configuration.
In addition, a switching device 103 for driving the unit pixel is formed at every intersection of the gate lines 101 and the data lines 102.
A thin film transistor (TFT) is commonly used as a switching device, and includes a gate electrode, a source electrode, a drain electrode and a channel layer. The gate electrodes are connected to the gate lines 101, and the source electrodes are connected to the data lines 102.
In addition, pixel electrodes 104 for applying an electric field to a liquid crystal 110 are formed on the lower substrate 100, and an alignment layer (not shown) for alignment of the liquid crystal 110 is formed on the pixel electrode 104 over an entire surface of the lower substrate 100. An organic layer such as polyimide is commonly used as the alignment layer, and alignment of the liquid crystal is prepared through a rubbing in which the alignment layer is rubbed with a rubbing cloth or the like after having been applied.
In addition, spacers (not shown) for maintaining a uniform cell gap between the attached lower substrate 100 and upper substrate 150 are scattered on the alignment layer.
Also, a sealant (not shown) is formed along an outer edge of a pixel region of the lower substrate 100 in order to attach the upper substrate and the lower substrate and prevent leakage of an injected liquid crystal.
Next, a structure of an upper substrate 150 for displaying an image in color will now be described.
A black matrix 151 for cutting off unnecessary light proceeding from the lower substrate 100 is formed as a matrix type on the upper substrate 150, and a color filter for displaying an image in color is formed on the black matrix. In general, red, green and blue (R, G and B) sub-color filters 152 are formed as one group for each unit pixel.
An overcoat layer 153 for compensating a step difference of a color filter layer may be formed on the color filter layer.
A common electrode 154 for applying an electric field to the liquid crystal together with the pixel electrode 104 of the lower substrate 100 is formed on the overcoat layer 153, and an alignment layer (not shown) for alignment of the liquid crystal is formed on the common electrode 154. Spacers (not shown) for maintaining a cell gap between the upper substrate 150 and the lower substrate 100 may be formed on the alignment layer. The spacers are formed on one of the upper substrate 150 and the lower substrate 100.
A structure of a color filter substrate, an upper substrate of an LCD device, will now be briefly described with reference to FIG. 2.
As for the color filter substrate, a black matrix 202 is formed on a substrate made of a transparent material. The black matrix 202 may be an opaque metallic thin film or a chemical resin which cuts off unnecessary light proceeding from the TFT array substrate, the lower substrate of the LCD device.
The black matrix 202 is formed as a matrix type, corresponding to the gate lines arranged in a longitudinal direction and the data lines arranged in a horizontal direction. A color resin for displaying an image in color is formed at each pixel region defined by the black matrix. Three different color resins (R, G and B color resins) are formed as one group for each unit pixel.
In addition, to compensate a step difference of the color filter and protect the color filter layer, a transparent overcoat layer 204 may be further formed on the color filter layer 203.
A common electrode 205, a transparent electrode for applying an electric field to a liquid crystal, is further formed on the overcoat layer 204, and spacers 206 for maintaining a cell gap of the LCD device are formed on the common electrode 205.
In addition, an alignment layer 207 for an alignment of the liquid crystal injected between the color filter substrate and the TFT array substrate is further formed on the spacer 206.
Next, a fabrication process of the color filter substrate of the LCD device adopting such a structure will now be briefly described with reference to FIGS. 3A to 3D.
First, a layer for forming a black matrix made of a metallic material or a resin material is formed on a transparent substrate.
In general, the black matrix is formed between the R, G and B sub-color filters in order to cut off light passing through a reverse tilt domain formed around the pixel electrode of the lower TFT array.
In general, a metallic thin film of chrome (Cr) or the like, the optical density of which is greater than 3.5, or an organic material such as carbon or the like is mainly used as the black matrix. To achieve low reflection, double layers such as chrome/chrome oxide (Cr/CrOx) may be used as the black matrix.
If the metallic thin film is used as the black matrix, a pattern may be formed on the metallic thin film by photolithography, and if a resin made of a photosensitive organic material is used as the black matrix, the pattern may be formed on the resin by an exposure process and a development process.
FIG. 3A shows a black matrix 202 having a pattern and formed on a substrate 201. In order to form the black matrix on the substrate, a first mask including a pattern for forming a black matrix is required.
After the black matrix has been formed 202, as shown in FIG. 3B, a color filter layer 203 including R, G and B sub-color filters 203a, 203b, and 204c, respectively, for displaying an image in color is formed.
The color filter may be fabricated by various methods such as a dyeing method, an electrodepositing method, a pigment dispersing method, a printing method or the like. As one example, a process for fabricating a color filter by the pigment dispersing method will now be described.
First, one of R, G and B color resins is applied on an entire surface of the substrate 201 on which the black matrix 202 has been formed (herein, the color resins are applied in order of R, G and B, but the applying order of the color resin can be set randomly). Then, exposure is selectively performed on the applied R color resin, thereby forming an R sub-color filter layer 203 at a desired region.
Then, the G color resin is applied on the substrate on which the R sub-color filter layer has been formed, and selective exposure is performed on the applied G color resin, thereby patterning a G sub-color filter layer 203b at a corresponding region. The same process is performed on the B color resin, thereby forming a B sub-color filter layer 203c. That is, when the color filter layer is formed, an exposure process is repeatedly performed using a second mask.
That is, in order to form the R, G and B sub-color filters, a mask process including exposure and development, and cleaning is performed three times.
After the color filter layer 203 has been formed, as shown in FIG. 3C, a transparent overcoat layer 204 of an organic layer component is formed in order to compensate a step difference of the color filter layer.
After the overcoat layer 204 has been formed, an ITO (Indium Tin Oxide) film 205, a transparent electrode for applying an electric field to a liquid crystal layer is further formed. The ITO film works as a common electrode 205.
Spacers 206 are formed on the common electrode 205 to maintain a uniform cell gap of the LCD device. In order to form the spacer, a scattering method in which ball type spacers are scattered or a patterning method in which a size, a height and a position of the spacer can be determined may be used.
The scattering method may be divided into a wet scattering method in which spacers are mixed with alcohol or the like and then scattered and a dry scattering method in which only spacers are scattered. Also, as the dry scattering method, there are a dry scattering method using static electricity and a non-electrostatic scattering method using air pressure, and the non-electrostatic scattering method is commonly used for a liquid crystal cell structure which is vulnerable to static electricity.
However, in the scattering method, the position and the height of the scattered spacer may not be determined. Therefore, a column spacer formation method which can increase an aperture ratio is being used.
In the column spacer formation method, a photosensitive resin for forming a spacer is applied on the common electrode, the applied resin is exposed by using a mask, and then the exposed resin is developed and cleaned, thereby forming a certain pattern. At this time, a mask process is further required.
After the spacer has been formed on the common electrode, an alignment layer is formed by depositing an organic layer made of polyimide or the like and rubbing the organic layer in a desired direction.
The color filter substrate of the LCD device is completed after completing these processes.
However, when the color filter substrate according to the related art is fabricated, many mask processes are required, which causes process delay and a decrease in productivity. Also, in one mask process, a series of processes such as deposition of a photosensitive layer, exposure of the photosensitive layer, cleaning of the photosensitive layer and the like are performed. For this reason, by reducing one mask process, productivity may be greatly improved and fabrication costs of the LCD device may be remarkably reduced.